Multi-leg exhaust aftertreatment system and method

ABSTRACT

An exhaust aftertreatment system for treating exhaust flow from an internal combustion engine, and associated method, allows for independent control of exhaust flow through plural exhaust legs of the exhaust aftertreatment system. The independent control of exhaust flow is carried out by adjusting a valve positioned in each the exhaust legs based on a value of a signal generated by a flow measurement device positioned along at least one of the exhaust legs. The valves can be adjusted to force a target flow in a exhaust leg, relative flow among exhaust legs, exhaust temperature in an exhaust leg, exhaust backpressure and/or imbalance within the exhaust legs.

TECHNICAL FIELD

The inventions relate to aftertreatment systems and methods for internalcombustion engines and, more particularly, to systems and methods inwhich exhaust flow is measured and controlled through multiple exhaustlegs of an exhaust after-treatment system.

BACKGROUND

Nitrogen oxides (NOx), which include nitric oxide (NO) and nitrogendioxide (NO₂), are formed during the high temperature and pressurecombustion of an air and fuel mixture in an internal combustion engine.These oxides cause a number of concerns related to the environment, suchas a source of ground-level ozone or smog, acid rain, excess aqueousnutrients, and can readily react with common organic chemicals, and evenozone, to form a wide variety of toxic products. Since the 1970's,government legislation has required increasing reductions of NOx inexhaust gas emissions.

To comply with increasingly stringent government mandates, industry hasdeveloped several NOx reduction technologies to treat post combustionexhaust, of which diesel oxidation catalyst (DOC) and selectivecatalytic reduction/reducer (SCR) technologies are actively pursued.

In addition to NOx reduction, governments have been imposing progressivemandates for reducing amounts of particulate matter (PM) in exhaustemissions. The diesel particulate matter filter (DPF) has been developedfor exhaust aftertreatment systems to remove diesel particulate mattercontaining soot, unburned fuel, lubrication oil etc. from the exhaustgas.

A DPF typically includes a filter encased in a canister that ispositioned in the diesel exhaust stream. The filter is designed tocollect PM while allowing exhaust gases to pass through it. Types ofDPFs include ceramic and silicon carbide materials, fiber woundcartridges, knitted fiber silica coils, wire mesh and sintered metals.DPFs have demonstrated reductions in PM by up to 90% or more and can beused together with a DOC to reduce HC, CO, and soluble organic fraction(SOF) PM in diesel exhaust.

SUMMARY

The inventions are directed to exhaust aftertreatment systems and amethod of exhaust aftertreatment that allow for independent control ofexhaust flow through each of plural exhaust legs in the exhaustaftertreatment system. At least one of the exhaust legs includes a flowmeasurement device configured to sense a characteristic of the exhaustflow in that leg, from which amounts of exhaust flow in each of theplural exhaust legs can be independently controlled based on the sensedflow characteristic. The exhaust valves can be adjusted to force atarget amount of exhaust gas flowing in an exhaust leg, a targettemperature of exhaust gas flowing in an exhaust leg, to increaseexhaust backpressure, and/or to force a balance or imbalance of exhaustflow among the plural exhaust legs.

In accordance with an embodiment consistent with the claimed invention,an exhaust aftertreatment system for treating exhaust flow from aninternal combustion engine includes a first exhaust leg positioned toreceive the exhaust flow from the engine, a first selective catalyticreducer (SCR), a first reductant dosing system including a doserupstream of the first SCR and configured to inject reductant into anexhaust stream in the first exhaust leg, and a first exhaust flowcontrol valve positioned along said first exhaust leg. A second exhaustleg of the exhaust aftertreatment system is positioned to receive theexhaust flow from the engine in parallel to the exhaust flow in thefirst exhaust leg. The second exhaust leg includes a second SCR, asecond reductant dosing system including a doser upstream of the secondSCR and configured to inject reductant into an exhaust stream in thesecond exhaust leg, and a second exhaust flow control valve positionedalong the second exhaust leg. The aftertreatment system includes a flowmeasurement sensor device positioned along at least one of the first andsecond exhaust legs and configured to generate a signal indicative of acharacteristic of exhaust mass flow or exhaust volume flow of theexhaust steam in that leg. A control module is configured toindependently control the first and second exhaust valves based on thesignal indicative of exhaust mass flow or exhaust volume flow.

In accordance with another embodiment consistent with the claimedinvention, a method of exhaust aftertreatment is provided for an exhaustaftertreatment system of an internal combustion engine. The exhaustaftertreatment system includes plural exhaust legs in parallel with oneanother, and each of the exhaust legs includes a selective catalyticreducer (SCR) and a controllable exhaust valve. The method includesflowing exhaust gas through the plural exhaust legs, detecting anexhaust gas flow characteristic in at least one of the plural exhaustlegs, determining amounts of the exhaust gas flow in the plural exhaustlegs based on said detected exhaust gas flow characteristic, andindividually adjusting the exhaust valves in the plural exhaust legsbased on the determined amounts of exhaust gas flow.

In yet another embodiment consistent with the claimed invention, amulti-leg exhaust aftertreatment system includes plural parallel exhaustlegs adapted to receive exhaust from an internal combustion engine. Eachof the exhaust legs includes a diesel oxidation catalyst (DOC), a dieselparticulate filter (DPF) and an independently controllable exhaustvalve. An exhaust flow sensor is positioned along at least one of theplural exhaust legs and is configured to sense an amount of exhaustflowing in that exhaust leg and generate a exhaust flow signalindicative of said amount. An exhaust valve control module is connectedto the exhaust flow sensor to receive the exhaust flow signal andgenerate control signals for each of the independently controllableexhaust valves based on the exhaust flow signal.

The various aspects are described hereafter in greater detail inconnection with a number of exemplary embodiments to facilitate anunderstanding of the invention. However, the invention should not beconstrued as being limited to these embodiments. Rather, theseembodiments are provided so that the disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a multi-leg after-treatmentsystem according to an exemplary embodiment in which each exhaust legincludes an exhaust flow control valve and an SCR.

FIG. 2 is a flowchart of a process according to an exemplary embodiment.

FIG. 3 is a schematic block diagram of a multi-leg after-treatmentsystem according to an exemplary embodiment in which each exhaust legincludes an exhaust flow control valve, a DPF and an SCR.

FIG. 4 is a schematic block diagram of a multi-leg after-treatmentsystem according to an exemplary embodiment in which a single SCR isprovided downstream exhaust legs, and each exhaust leg includes anexhaust flow control valve, a DOC and a DPF.

DETAILED DESCRIPTION

The inventors have recognized that exhaust flow in individual legs of amulti-leg exhaust aftertreatment system can be known and controlled toprovide a balanced state of flow among the exhaust legs or to force animbalanced state of flow through the exhaust legs. To determine thestate of exhaust flow in the exhaust legs in the multi-leg exhaustaftertreatment system, at least one exhaust leg includes a mass flow orvolume flow measurement device that provides a measurement of acharacteristic of exhaust flow, in real time, through that exhaust leg.The measured characteristic can be a direct measurement of the exhaustflow or another characteristic from which the mass flow or volume flowcan be calculated for that exhaust leg and/or the relative exhaust flowbetween the legs can be determined. Control or compensation remedies canbe applied according to the determined state of exhaust flow and otheroperating parameters.

FIG. 1 is a block diagram showing an internal combustion engine 10fluidly connected with an exhaust aftertreatment system 12. The exhaustaftertreatment system 12 includes a main exhaust passage 14 and at leasttwo exhaust legs or passages 16 a and 16 b splitting off from the mainexhaust passage 14. The main exhaust passage 14 and exhaust legs 16 a,16 b can be connected downstream of a common exhaust manifold (notshown) attached to the engine 10. Also, engine 10 may include one ormore turbochargers (not shown). Each of the exhaust legs 16 a, 16 bincludes an SCR, a reductant dosing system and valve. Additionally, theexhaust aftertreatment system 12 includes a control module 26, which canbe, for example, an electronic control unit (ECU) or electronic controlmodule (ECM) that monitors the performance of the engine 10 and otherelements of the exhaust aftertreatment system 12. More specifically,FIG. 1 shows exhaust leg 16 a including an SCR 22 a, a diesel emissionsfluid (DEF) reductant dosing system 24 a, and an exhaust valve 30 a.Similarly, the exhaust leg 16 b includes an SCR 22 b, a DEF reductantdosing system 24 b, and an exhaust valve 30 b. Each exhaust leg 16 a and16 b also can have multiple temperature and pressure sensors (notshown), which could be placed ahead and behind each element in theexhaust leg to monitor temperature and pressure at various points alongthe legs 16 a and 16 b.

Each of the DEF dosing system 24 a and 24 b can include a doser, adecomposition reactor, and a mixer (not shown) to deliver a meteredamount of a reductant into the exhaust stream (flow) upstream the SCRdevice in each leg. The reductant can be an NH₃ source, such asanhydrous NH₃, aqueous NH₃, or a precursor that is convertible to NH₃such as urea ammonia or urea, which is stored in dosing treatment supply(not shown). In the SCR process, the reductant in the exhaust stream isabsorbed onto the SCR catalyst where it is used to convert NOx emissionsin the exhaust gas flow into nitrogen and water, and in the case ofurea, also into carbon dioxide. The predetermined amount of reductant tobe injected into one leg of the exhaust aftertreatment system 12 may bedelivered in a particular rate shape, such as disclosed in U.S. Pat. No.7,587,890, the entire contents of which is hereby incorporated byreference.

The control module 26 can be a single control unit or plural controlunits that collectively perform the monitoring and control functions ofthe exhaust aftertreatment system 12. The control module 26 utilizessensors to determine whether the exhaust aftertreatment system 12 isfunctioning properly. The control module 26 generates control signalsbased on information provided by sensors described herein and perhapsother information, for example, stored in a database or memory integralto or separate from the control module 26. The signal paths between thecontrol module 26, the sensors and other devices are depicted in FIG. 1using dashed lines. It is to be understood that these dashed signalpaths can be representative of either hard wired or wirelesscommunication paths.

The control module 26 can include a processor and modules in the form ofsoftware programs or routines executable by the processor of the controlmodule 26. These modules can be stored on tangible computer readablemedia such as memory positioned local to the control module 26 orlocated remote from, but accessible by the control module 26. Inalternative embodiments, modules of control module 26 can includeelectronic circuits for performing some or all or part of theprocessing, including analog and/or digital circuitry. The modules cancomprise a combination of software, electronic circuits andmicroprocessor based components. The control module 26 can receive dataindicative of engine performance and exhaust gas composition including,but not limited to engine position sensor data, speed sensor data,exhaust mass flow sensor data, fuel rate data, pressure sensor data,temperature sensor data from locations throughout the engine 10 and theexhaust aftertreatment system 12, requested speed or torque, NOx sensordata, and other data. The control module 26 can then generate controlsignals and output these signals to control various components in theengine 10 and system 12. For example, as shown in FIG. 1, the controlmodule 26 can be connected to each of the DEF dosing systems 24 a, 24 bto control an injecting device such as an atomizer (not shown) to injecta reductant into the exhaust stream in a respective leg 16 a, 16 bupstream of the respective SCR devices 22 a, 22 b. For example, thecontroller 26 can control a timing and amount of reductant injected intothe exhaust stream by each DEF dosing system 24 a, 24 b.

The control module 26 receives NOx sensor data by way of a NOx sensor 28provided in the main exhaust passage 14 upstream of the point where themain exhaust splits into legs 16 a, 16 b, to sense the NOx concentrationin the main exhaust and generate a signal indicative of the engine-outNOx concentration, although other ways to determine engine-out NOxconcentration can be used, such as by using a virtual NOx sensor.

As shown in FIG. 1, the exhaust valve devices 30 a, 30 b are provided inrespective exhaust legs 16 a, 16 b of the exhaust aftertreatment system12. Each exhaust valve 30 a, 30 b is controllable, such as an exhaustthrottle that is adjustable via an actuator controlled by control module26, and can be provided at a position in a leg after the point where themain exhaust passage 14 splits into the plural exhaust legs, althoughthe exhaust valves 30 a, 30 b can be provided at any point along arespective exhaust leg. Each exhaust valve device can be adjustedindividually and independently from any other exhaust valve in theexhaust aftertreatment system 12 such that the exhaust legs 16 a, 16 bcan assume any position from fully open, fully closed, and any partiallyopen state. Thus, the exhaust valves 30 a, 30 b can be individuallycontrolled to allow a particular amount of exhaust flow through therespective exhaust legs and control exhaust backpressure to the engine10 for general thermal management. The valve devices 30 a, 30 b alsoallow the exhaust stream flow from the main exhaust passage 14 to besplit unevenly between the between the two exhaust legs 16 a, 16 b.Further, the relative exhaust mass flow or volume flow among the exhaustlegs 16 a, 16 b can be measured, and the valve devices 30 a, 30 b can becontrolled to correct for a flow imbalance or to force an amount ofexhaust flow in a leg or relative flow among plural legs to apredetermined target value or ratio.

FIG. 1 shows exhaust gas flow measurement devices 34 a and 34 b fordetermining exhaust mass flow or volume flow in positions of respectiveexhaust legs 16 a and 16 b downstream respective exhaust valve devices30 a and 30 b. However, it is to be understood that the flow measurementdevices 34 a and 34 b can be positioned anywhere along a respectiveexhaust leg 16 a and 16 b. The exhaust gas flow measurement devices 34a, 34 b sense a mass or volume flow characteristic, such as pressure,from which mass flow in a leg can be determined. The flow measurementdevices 34 a, 34 b can be delta-P based devices, hot-film type devices,vortex shedding type devices, ultrasonic type devices, or any other typeof flow measurement device. Further, it is to be noted that while FIG. 1depicts the exhaust gas mass or volume flow measurement devices 34 a, 34b positioned in each of respective legs 16 a, 16 b, only one of the twoexhaust legs need be equipped with a flow measurement device because theremaining flow values can be computed or derived from other availableoperating information. For example, if an amount of exhaust flow throughone exhaust leg is known, the exhaust mass or volume flow through theremaining leg can be computed in a straightforward manner when the totalmass or volume measurement for the entire engine is known. One method isto measure the pressure difference across one or both SCR 22 a, 22 bsince the effective flow area of the SCR is expected to be constant overtime and have relatively minimal part-to-part variation.

Referring again to FIG. 1, the control module 26 includes a mass flow orvolume flow MASS/VOL module 32, which receives signals indicative of aflow characteristic from the flow measurement devices 34 a, 34 b anddetermines the exhaust gas flow (mass flow or volume flow) in eachrespective exhaust leg 16 a, 16 b.

In other exemplary embodiments, the exhaust gas flow (mass flow orvolume flow) amounts can be determined in other ways. For example, bymeasuring the ammonia concentration downstream of the DEF dosing systems24 a, 24 b, the MASS/VOL module 32 can determine exhaust flow in eachexhaust leg 16 a, 16 b from the total mass flow rate of the reductant(DEF) and the measured concentration. For instance, by injecting thesame amount of reductant (DEF) into both exhaust legs 16 a, 16 b, therelative split between each exhaust leg can be determined by monitoringthe relative concentrations of NH₃ in each exhaust path. This isconsidered an open loop system because no adjustment would be made tothe DEF to compensate for the flow imbalance. If NH₃ concentration ismeasure before the SCRs 22 a and 22 b, the NH₃ concentrations are higherand unaffected by the SCRs. This can provide a more direct measurebecause it removes sensitivity to SCR catalyst performance. However,higher NH₃ concentrations can lead to potential sampling issues due todeposit formation and obtaining an accurate measurement of NH₃ can bedifficult (e.g., sampling across the area of the exhaust pipe).

In another exemplary embodiment, exhaust gas flow amounts can bedetermined by measuring the ammonia concentration downstream of each ofthe SCRs 22 a and 22 b, or inside each of the SCRs, and then using theMASS/VOL module 32 to calculate relative amounts of exhaust flow in thetwo exhaust legs 16 a and 16 b. The MASS/VOL module 32 also cancalculate the actual flow amount in each of the exhaust legs 16 a and 16b knowing the DEF (reductant) flow rate for the each exhaust leg and thetotal mass flow rate through the engine 10 (e.g., from a speed densitycalculation or from an air mass flow measurement in the intake system).The measured ammonia concentration can be used as a feedback parameterto control each of the DEF dosing systems 24 a and 24 b. In this way,the amount of DEF being injected into each of the exhaust legs 16 a and16 b can be adjusted independently based on a close loop controlfeedback from the ammonia sensor, which is more robust compared with aflow split from an SCR point of view. If such feedback control is used,the outlet NH₃ concentration ends up being the same for both legs. Thiscan be achieved by adjusting the commanded DEF flow. The relative flowbetween the legs can be estimated based on the difference in commandedDEF flow. If the commanded flows are the same, then the flow isbalanced. If one leg is commanding higher DEF, then the mass flowthrough that leg is higher. There are assumptions built into thismethod, which include that the DEF dosers accurately deliver a commandedDEF quantity, the NH₃ concentration is measured accurately, the NOxdistribution is homogeneous, and that other components is the system arebehaving similarly. If feedback control is not used, then theconcentrations can be used directly to calculate the exhaust gas flowamounts.

In another exemplary embodiment, the MASS/VOL module 32 can calculatethe flow in each leg 16 a, 16 b, as follows: the air flow through theengine 10 is known from a speed-density calculation and fuel flow (e.g.,from a calculation performed by the ECM); from these parameters, thetotal air flow through the exhaust aftertreatment system 12 can bedetermined; and by measuring the imbalance (e.g., a 40/60 split), theexhaust flow or volume in each leg 16 a, 16 b can be calculated. Oneexemplary way to measure the imbalance in the exhaust flow through theexhaust legs is to measure the pressure difference between the two legs16 a, 16 b in different locations in the aftertreatment system. Forexample, measuring a difference in pressure between the inlets of eachSCR 22 a, 22 b of the respective exhaust legs 16 a, 16 b can provide agood indication of the exhaust flow imbalance because the inlet and theoutlet pressures for the two complete aftertreatment subsystems (i.e.,complete exhaust legs 16 a, 16 b) are the same. An algorithm utilized bythe MASS/VOL module 32 in this example can access a table (e.g., alookup table pre-stored in memory) that was created through empirical oranalytical testing/analysis to calculate the relative flow split basedon this delta-p measurement, the known total flow through the system,the inlet pressure and temperature, and other pressure and temperaturemeasurements in the system. The algorithm also can calculate the flowsplit using equations based on principles of physics. After the exhaustflow imbalance is calculated, this information can be used in manydifferent ways. For example, the control module 26 can compensate thereductant dosing in each leg 16 a, 16 b to provide a correct amount ofreductant.

In the exemplary embodiment shown in FIG. 1, a NOx feedback sensor 40 aand a temperature sensor 42 a are positioned downstream of the exits ofthe SCR catalyst 22 a, and a NOx feedback sensor 40 b and a temperaturesensor 42 b are positioned downstream of the exits of the respective SCRcatalyst 22 b. While not shown in FIG. 1, each exhaust leg 16 a, 16 bcan include a reductant sensor (e.g., an NH₃ sensor) provided downstreamof respective SCR catalysts 22 a, 22 b as an alternative to, or inaddition to a NOx feedback sensors 40 a, 40 b.

Accordingly, determining the exhaust flow and individually controllingthe exhaust flow in exhaust legs 16 a, 16 b via an exhaust throttle canimprove the performance of an SCR system. The addition of temperaturecontrol in a plural exhaust leg system makes it possible to raise thetemperature of the SCR system at light loads, which will result inimproved conversion efficiencies and lower system-out NOx. This isparticularly beneficial in applications where the engine spends a lot oftime at light load. For example, by completely closing an exhaust valvein one exhaust leg of an aftertreatment system, the percent heat lossthrough an open exhaust leg would be reduced because of reduced crosssectional area. DEF delivery in an aftertreatment system could also bemore accurate. SCR performance can be improved by forcing exhaust flowentirely, or substantially entirely through one leg because thecatalysts in the leg with the flow would be at a higher averagetemperature than if flow went through two legs due to reduced fractionof heat loss to the catalysts (the surface area for heat loss is cutapprox. in half). The flow through the other leg would be zero orsubstantially zero (close to zero). The exhaust leg without flow wouldcool down slowly because there would be no internal flow to take awaythe heat. However, under extended operation at light load, the exhaustleg having no, or substantially no exhaust flow would eventually cooldown. To address this scenario, a scheme according to an embodiment canswitch exhaust flow periodically between the exhaust legs to keep theentire system warm so that when load is applied, both legs are thermallyready to perform efficiently. It also would be possible, if required, topartly close an exhaust valve in an open exhaust leg with the idea ofincreasing the work that the engine has to do, which would furtherincrease the exhaust temperature. During high load operation the flowthrough the system will be relatively equally split between the twolegs, minimizing exhaust backpressure, and allowing the engine todeliver rated power while reducing the temperature in the SCR system.(At high loads the high exhaust temperatures typically results in SCRcatalyst temperatures which are higher than optimum).

Thermal control by way of independent adjustment of exhaust valves alsocan manage condensed hydrocarbon/water present or accumulating in theSCR catalysts (150 C for water, 220 C for hydrocarbon). SCR performancewould likely improve at the same time. Additionally, embodiments canaddress uncertainties in the exhaust flow rates that can result inerrors in the ammonia-to-NOx ratio (ANR), which negatively affect theSCR conversion efficiency or increase ammonia slip.

FIG. 2 shows an exemplary process flow 50 for individually controllingexhaust flow from an engine through legs in a multi-leg exhaustaftertreatment system, such as exhaust aftertreatment system 12,including individually controllable valves in each of the exhaust legs.While the process shown in FIG. 2 is described with respect to anexhaust system including two exhaust legs, it is to be understood thatsimilar process can be applied to an aftertreatment system having morethan two exhaust legs.

Process flow 50 starts at process 52, exhaust gas from an internalcombustion engine is flown though a first exhaust leg and a secondexhaust leg. Next, process 54 detects an exhaust mass flow or volumeflow characteristic in the first or second leg. In process 56, theexhaust flow in each leg is determined using the detected flowcharacteristic, and/or the relative flow between the exhaust legs isdetermined In process 58, the valves in the exhaust legs areindividually controlled to control an amount of exhaust through each ofthe exhaust legs.

FIG. 3 shows an exhaust aftertreatment system 112 according to anembodiment. Items of exhaust aftertreatment system 112 having the samenumbers as in the exhaust aftertreatment system 12 are described above.Additionally, items of the exhaust aftertreatment system 112 shown inFIG. 3 that are similar to those described above have reference numbersthat are 100 more in count than corresponding item reference numbersshown in FIG. 1.

As shown in FIG. 3, the exhaust aftertreatment system 112 is fluidlyconnected to an internal combustion engine 10 by a main exhaust passage14 that splits into plural exhaust legs or passages 116 a, 116 b,although in other embodiments the plural exhaust legs 116 a, 116 b canfluidly connect to an exhaust manifold of the engine (not shown) withoutincluding the main exhaust 14 segment therebetween. Additionally, it isto be understood that an embodiment can include more than the twoexhaust legs.

The exhaust aftertreatment system 112 is a DOC/DPF/SCR aftertreatmentarchitecture that processes exhaust from the internal combustion engine10 using the plural exhaust legs 116 a, 116 b. More specifically, theexhaust leg 116 a includes a DOC 118 a, a DPF 120 a, an SCR 122 b, and aDEF dosing system 124 a. Similarly, the exhaust leg 116 b includes a DOC118 b, a DPF 120 b, an SCR 122 b, and a DEF dosing system 124 b.

Different DPF designs could be used, including but not limited to wallflow, partial flow, catalyzed, or non-catalyzed. Because the DPFs 120 a,120 b trap soot and other PM in the exhaust stream, these particles canaccumulate and plug a DPF in a relatively short time. To prevent theexhaust gas passages in the DPFs from becoming constricted or plugged,these filters must be regenerated from time-to-time to burn off or“oxidizes” accumulated PM. With diesel internal combustion engines,exhaust temperatures often are not sufficiently high to burn accumulatedPM so regeneration can includes raising the exhaust gas temperatureand/or lowering the oxidation temperature to promote oxidation of thePM.

Regeneration of the DPFs 120 a, 120 b can be accomplished in a passiveand/or active mode. For example, a passive regeneration scheme caninclude adding a catalyst to the DPF to lower oxidation temperature. Inan embodiment, a base or precious metal coating can be applied to thefilter surface to reduce the ignition temperature required for oxidizingaccumulated PM. Additionally, the DOCs 118 a, 118 b provided upstream ofa respective catalyzed DPF 120 a, 120 b can include a catalyst topromote oxidation of CO and HC emissions, SOF, and NO in the exhauststream. NO oxidizes to generate NO₂, which is a potent oxidizer of PM inthe DPFs 120 a and 120 b downstream of the respective DOCs 118 a, 118 b.Examples of active regeneration schemes that increase the exhaust gastemperature include injecting fuel into the exhaust stream, engineoperation management, applying heat (e.g., resistive coils) to theexhaust stream, a fuel burner, and/or late fuel injection. The oxidizingprocess in the DOCs 118 a, 118 b requires accurate control to maintainthe mass ratio of NO/PM in engine-out exhaust gas. Active systems canuse pulses of diesel fuel late in the combustion cycle to oxidize acrossthe catalyst thereby heating the DPF 120 a, 120 b and oxidizing trappedPM. However, running the cycle too often while keeping the back pressurein the exhaust system low, can result in excess fuel use. As will bedescribed later in detail, an active regeneration scheme of a DPFincludes exhaust valves 130 a, 130 b that allow for individual controlof exhaust gas flow through exhaust legs 116 a, 116 b of the exhaustaftertreatment system 112.

Each SCR 122 a, 122 b is positioned downstream of the respective DPF 120a, 120 b for removing NOx emissions from the exhaust gas. Respectivediesel emissions fluid (DEF) dosing system 124 a, 124 b is providedupstream the SCR devices 122 a, 122 b to selectively and controllablyprovide a dose of a reductant to the exhaust stream in the exhaust legs.Refer to the above embodiment shown in FIG. 1 for a more detailed SCRdescription.

The exhaust aftertreatment system 112 includes a control module 126,which can be implemented as the control module 26 described above, butthe control module is also capable of receiving a signal indicative ofthe engine-out NOx concentration sensed by NOx sensor 28. The engine-outNOx concentration signal is provided to a soot loading estimator (SLE)module 150, which can be implemented as a module in the control module126, as shown in FIG. 3. Alternatively, an SLE module can be implementedseparate from the control module 126, to estimate soot load in each ofthe DPFs 120 a, 120 b.

The SLE module 150 determines soot loading estimates, which can providean estimate as to when regeneration of a DPF in an exhaust leg shouldoccur. However, soot loading of the DPFs 1120 a, 120 b of the pluralexhaust legs 116 a, 116 b can vary from leg-to-leg. These leg-to-legvariations occur because soot is loaded as a function of exhaust massflow or exhaust volume flow, and the exhaust legs 116 a, 116 b can havedifferent geometry either by design or manufacturing tolerances.Additionally, on the engine 10, bank-to-bank differences can createdifferent input conditions to different exhaust legs. For instance, fuelinjector variability on the engine side could drive PM differencesbetween the banks. Thus, knowing the exhaust flow in an exhaust legand/or the relative flow between exhaust legs in the multi-leg exhaustaftertreatment system 112 can aid the SLE module 150 in estimating withgreater accuracy a soot loading of a DPF, and thus when to regeneratethat DPF.

As shown in FIG. 3, each leg 116 a, 116 b of the exhaust aftertreatmentsystem 112 is provided with a respective valve device 130 a, 130 b, suchas an adjustable exhaust throttle. The exhaust valve devices 130 a, 130b are positioned in the exhaust leg after the point where the mainexhaust passage 14 splits into the plural legs, although the valves 130a, 130 b can be positioned anywhere along its respective exhaust leg.The valve devices 130 a, 130 b are individually controllable to allowfor selectively increasing backpressure to the engine 10 for generalthermal management. The valve devices 130 a, 130 b also allow theexhaust stream flow from the main exhaust passage 14 to be splitunevenly between the two legs 116 a, 116 b. Further, the relativeexhaust mass flow or volume flow among the legs 116 a, 116 b can bemeasured, and the valve devices 130 a, 130 b can be controlled tocorrect for a flow imbalance or forcing a flow amount in an exhaust legor relative flow among plural legs to a target value or ratio.

The control module 126 includes a mass flow or volume flow MASS/VOLmodule 132, which receives signals from flow (or volume flow)measurement devices 134 a, 134 b. The flow measurement devices 134 a,134 b are positioned in respective legs 116 a, 116 b and sense anexhaust mass flow or volume flow, or one or more characteristic of theexhaust stream from which the MASS/VOL module 132 can derive the exhaustmass flow or volume flow. The volume flow measurement devices 134 a, 134b can be implemented in any of the manners described above andpositioned anywhere along a respective leg 116 a, 116 b. While FIG. 3depicts mass flow or volume flow measurement devices 134 a, 134 bpositioned in each of the exhaust legs 116 a, 116 b, if a reliable totalmass measurement for the entire engine is available, a mass flowmeasurement device can be included in only one of the two exhaust legsof the exhaust aftertreatment system 112 because the mass or volume flowthrough the remaining leg can be computed from these values in astraightforward manner.

The exhaust aftertreatment system 112 can include hydrocarbon dosers 136a, 136 b positioned upstream of respective DOCs 118 a, 118 b in eachrespective leg 116 a, 116 b. The hydrocarbon dosers 136 a, 136 b injecthydrocarbons (e.g., fuel) into exhaust stream in an exhaust leg tocontrollably increase the temperature of the exhaust in that particularleg. Each exhaust leg 116 a, 116 b also can have multiple temperatureand pressure sensors (not shown), which could be positioned at pointsupstream and downstream of each element in the leg to monitortemperature and pressure at those points.

After calculating the exhaust flow or imbalance in the legs 116 a, 116b, this exhaust flow information can be used in several different ways.For example, the control module 126 can compensate the reductant (e.g.,urea) dosing in each leg 116 a, 116 b to provide a correct amount ofreductant. The diagnostics algorithm also can draw conclusions about thehealth of the system, and can detect whether any of the DPFs 120 a and120 b are plugged or damaged. This can be based on knowledge that overthe long run, soot loading in each leg 116 a, 116 b will balance to givevery similar flows in each leg. If a significant flow-imbalance isdetected, and this imbalance persists over time, it can reliably beconcluded that there is a DPF failure. By comparing the pressure dropacross a DPF to an expected value, for example, based on a delta-p basedsoot load estimation provided by the SLE 150, the diagnostics candetermine if a failure is related to the DPF being plugged (i.e.,pressure drop is higher than anticipated), or whether the DPF is cracked(i.e., pressure drop is lower than anticipated). The flow measurementdevices 134 a and 134 b supply NOx flux information to a reductantdosing algorithm executed by respective DEF dosing systems 124 a, 124 b,which attempt to deliver a targeted ANR. The flow measurement devices134 a and 134 b also can supply exhaust flow input to the soot loadestimation algorithm implemented in the SLE 150.

As shown in FIG. 3, an ammonia oxidation (AMOX) catalyst device 138 a,138 b can be positioned downstream of a respective SCR 122 a, 122 b, toreduce ammonia slipping past the SCRs, although an AMOX catalyst is notrequired in this and other embodiments. Each AMOX catalyst device 138 a,138 b can be an integral part of the upstream SCR 122 a, 122 b in anexhaust leg 116 a, 116 b, or a separate container connected in the legs116 a, 116 b downstream of the SCRs. Downstream of the AMOX is theexhaust stack (not shown). In the exemplary embodiment shown in FIG. 3,a NOx feedback sensor 140 a, 140 b and a temperature sensor 142 a, 142are placed downstream of the outlets of respective AMOX catalyst devices138 a, 138 b. Alternatively, or in combination with the NOx sensors 140a, 140 b, a reductant sensor such as an ammonia sensor (not shown) canbe positioned after the AMOX to measure a reductant slip concentrationexiting the system and provide feedback to the DEF dosing system 224 andcontrol module 126 for appropriate ANR adjustment based on thisfeedback.

The control module 126 includes a regeneration (REGEN) module 160 thatutilizes a soot load estimated by the SLE 150 and individually managesexhaust flow and temperature in each of the two legs 116 a, 116 b tocarry out regeneration of the DPFs in the exhaust leg 116 a, 116 b. Atlight loads, it can be advantageous to pass most or all of the exhaustflow through one or more of the exhaust legs, but not through others.For example, the REGEN module 160 can cause one of the two legs 116 a,116 b to intentionally have a different flow from the other leg. In someembodiments, for example, the optimal split between the exhaust legs 116a/116 b can vary between split ratios of 50/50 to 0/100 to 100/0,depending on the operating condition, although any split ratio can beachieved with an appropriate control of valve devices 130 a, 130 b. Forexample, a 0/100 split ratio can be accomplished by closing exhaustvalve 130 a of leg 116 a entirely and leaving exhaust valve 130 b of leg116 b open to direct substantially the entire exhaust stream from theengine 10 though the exhaust leg 116 b.

The REGEN module 160 can individually control exhaust valves 130 a, 130b to provide a minimum amount of exhaust flow required to regenerate theDPF 120 a or 120 b of an exhaust leg 116 a or 116 b, and thus provide away to control and initiate a regeneration event in a particular exhaustleg. In other embodiments, a multi-leg exhaust aftertreatment systemconsistent with the claimed invention can include more than two exhaustlegs, and more than two exhaust valves can be individually controlled toinitiate regeneration in one or plural DPFs. Also, regeneration canoccur at lower loads by passing more than of the exhaust through onesubset of the exhaust legs than through another subset of the exhaustlegs, for example, more than half of the exhaust through one of the twoexhaust legs 116 a, 116 b shown in FIG. 3. Additionally, restricting theflow in this manner can controllably increase the backpressure on theengine 10, which increases the load on the engine and thus the exhausttemperature to aid in regeneration. Further, if only one leg isregenerated at a time (for example, at light load), passing most or allof the flow through one exhaust leg controllably increases the rate atwhich the DPF heats up to allow for faster regeneration at light loadsusing less hydrocarbon dosing, which provides a increased efficiencywith respect to fuel consumption.

The regeneration algorithm of the REGEN module 160 attempts to achieve atarget temperature at the outlet of the DOCs 118 a, 118 b. By adding theability to adjust the relative flow through the two legs 116 a, 116 b,the target temperature at the DOC outlet of an exhaust leg that is beingregenerated can be adjusted more effectively. At the beginning of a DPFregeneration event, higher exhaust flows tend to increase the rate atwhich the DPF temperature climbs. Once the temperature in the DPF hasreached a level at which oxidation of the particulates is occurring, thetrend is reversed, and lower flow rates lead to higher temperatures, asthe heat generated by the reaction cannot be carried away aseffectively.

In some embodiments, at low system loads, a passive regeneration eventcan be initiated by channeling most or all of the exhaust flow to anexhaust leg that is to be regenerated. This will increase the exhausttemperature in that exhaust leg due to the higher backpressure. Ahydrocarbon source (e.g., fuel) can be injected into the exhaust streamof an exhaust leg via the hydrocarbon dosers 136 a, 136 b to furtherincrease the exhaust temperature, allowing the oxidation of particulatein the DPF in that exhaust leg to start. Once the temperature in the DPFhas reached the ideal range, hydrocarbon dosing is no longer needed andthe temperature range can now be adjusted by controlling the exhaustflow through the DPFs 120 a, 120 b via respective exhaust valves 130 aand 130 b. Once the regeneration of one exhaust leg has been completed,the system can switch to regenerate another exhaust leg.

In another embodiment, during light loads, the exhaust valve devices 130a, 130 b can be adjusted to positions where most of the exhaust flow ispassed through more than one leg to increase back pressure, and thusalso increase exhaust temperature to permit passive regeneration of theDPFs positioned in those exhaust legs. Moreover, the DPF temperature canincrease further because the higher flow rate results in a smallerpercentage of heat lost due to convection and radiation. By avoiding orreducing injection of hydrocarbon into the exhaust stream, greater fuelefficiency can be obtained.

As can be seen, dynamically adjusting the split between two or moreexhaust legs (e.g., exhaust legs 116 a and 116 b) can allow forcontrollable efficient regeneration events. In addition to controllingthe relative flow through the two exhaust legs, it is also possible toclose both exhaust throttles in such a way to control both the split andthe backpressure. This additional control lever can also be used tofurther optimize a regeneration event.

In a case where excessive DPF temperature exists in one of the exhaustlegs, the control module 126 can be configured to control an exhaustflow control device such as exhaust valves 130 a or 130 b to completelyshut off the exhaust flow to the exhaust leg having the DPFover-temperature, which can protect that DPF from damage in such asituation. With shutoff, the flow of oxygen to the DPF is cut off andthe particulate oxidation will stop, thus preventing an undesiredover-temperature event.

FIG. 4 is a diagram of a multi-leg exhaust aftertreatment system 212according to another exemplary embodiment. The multi-leg aftertreatmentsystem 212 is similar to the exhaust aftertreatment system 112 shown inFIG. 3 in that an exhaust passage 14 from an internal combustion engine10 is split to form plural legs 216 a, 216 b, each of which includes anexhaust valve 130 a or 130 b, a DOC 118 a or 118 b, and a DPF 120 a or120 b, as described above, but the exhaust legs in system 212 arecombined into a single exhaust channel 170 downstream the DPFs 120 a and120 b. A single SCR 222 and DEF dosing system 224 are positioned alongthe channel 170. In the multi-leg exhaust aftertreatment system 212, theflow through the DEF dosing system 124 and the SCR 222 can be accuratelycalculated from the total mass flow through the engine, which allowsaccurate DEF dosing. Although not shown, the system 212 can include anAMOX catalyst downstream of the SCR 222 to reduce ammonia slip.

Under normal operating conditions, the SCR element 222 is the primaryelement of the exhaust aftertreatment system for removing NOx from theexhaust gas stream. During normal operating conditions, the controlmodule 226 receives signals from various sensors, such as the throttlesensor and air temperature sensor on the engine (not shown) or a NOxsensor 140 provided after the exit of the SCR 222. From the sensedconditions, the controller 126 can determine various parameter values ofengine 10 and the exhaust aftertreatment system 212. More specifically,the controller 226 will control the DEF doser system 224 to injectreductant at a rate needed to operate the SCR 222 for a currentoperating condition of the engine 10 and in view of any measured orestimated imbalance in exhaust mass flow or volume flow. Alternatively,or in combination with the NOx sensor 140, a reductant sensor 142, suchas an ammonia sensor, can be provided to measure the reductantconcentration downstream of the SCR 222 and provide feedback to the DEFdosing system 224. Using feedback information from the ammonia sensor142 and/or the NOx sensor 140, the DEF dosing system 224 can determinean appropriate ANR.

It will be appreciated that the embodiments described and shown hereinmay be modified in a number of ways. For instance, although theexemplary embodiments described above each include two legs, a multi-legexhaust aftertreatment system consistent with the claimed invention caninclude more than two legs, and control of a balance or unbalanced themass flow and volume flow through each of the legs can be carried outfor each exhaust leg. Further, prior to assembly, the mass flow orvolume flow through one or more elements constituting a leg can bemeasured and control to affect differential mass flow or volume flowamong the exhaust legs can be carried out based on the measured values.

Although not shown, an exemplary embodiment can include temperaturesensors positioned at the entrance and the exit of the SCR in each legof a multi-leg exhaust aftertreatment system consistent with the claimedinvention. By dynamically measuring these temperatures and combiningthis information with a model of the thermal behavior of the SCRcatalyst, it is possible to estimate the relative flow in the differentlegs.

In another exemplary embodiment, the complete system is modeled and theconversion efficiency of the system is measured at the exit of eachexhaust leg. From this information, the relative flow in the differentexhaust legs can be estimated.

Additionally, the exhaust aftertreatment system can include otherelements such as a NOx adsorber catalyst, or lean NOx trap (LNT)implemented in a separate chambers from the SCR or both the SCR and LNTimplemented in a same chamber.

Furthermore, while the exemplary embodiments are sometimes describedherein in the context of a diesel internal combustion engine, the sameconcepts can be applied in a lean burn gas, such as a lean burn gasolineor natural gas, powered internal combustion engine.

Embodiments consistent with the claimed invention facilitate balancingexhaust flow through plural exhaust legs of a multi-leg exhaustaftertreatment system, which can substantially equalize soot loadingamong the DPFs of the exhaust legs. Additionally, the flow in theexhaust legs can be controlled to actively drive a flow imbalance tocompensate for flow differences resulting from manufacturingvariability, differences by design and/or performance differences amongthe exhaust legs and/or to initiate regeneration in a DPF of a leg.

Although a limited number of exemplary embodiments is described herein,one of ordinary skill in the art will readily recognize that there couldbe variations to any of these embodiments and those variations would bewithin the scope of the appended claims. Thus, it will be apparent tothose skilled in the art that various changes and modifications can bemade to the multi-leg exhaust aftertreatment system and method describedherein without departing from the scope of the appended claims and theirequivalents.

1. An exhaust aftertreatment system for treating exhaust flow from aninternal combustion engine, comprising: a first exhaust leg positionedto receive the exhaust flow from the engine, said first exhaust legincluding: a first selective catalytic reducer (SCR); a first reductantdosing system including a doser upstream of the first SCR and configuredto inject reductant into an exhaust stream in the first exhaust leg; anda first exhaust flow control valve positioned along said first exhaustleg; a second exhaust leg positioned to receive the exhaust flow fromthe engine in parallel to the exhaust flow in the first exhaust leg,said second exhaust leg including: a second SCR positioned along thesecond exhaust leg; a second reductant dosing system including a doserupstream of the second SCR and configured to inject reductant into anexhaust stream in the second exhaust leg; and a second exhaust flowcontrol valve positioned along the second exhaust leg; a flowmeasurement sensor device positioned along at least one of the first andsecond exhaust legs and configured to generate a signal indicative of acharacteristic of exhaust mass flow or exhaust volume flow of theexhaust steam in that leg; and a control module configured toindependently control the first and second exhaust valves based on saidsignal indicative of exhaust mass flow or exhaust volume flow.
 2. Theexhaust aftertreatment system according to claim 1, wherein the controlmodule determines relative amounts exhaust flow present in each of thefirst and second exhaust legs.
 3. The exhaust aftertreatment systemaccording to claim 1, wherein a flow measurement sensor device ispositioned in each of the first and second exhaust legs.
 4. The exhaustaftertreatment system according to claim 3, wherein each flowmeasurement sensor device is a pressure sensor that generates a pressuresignal indicative of an exhaust pressure, and the electronic controlmodules determines a delta-p measurement from the generated pressuresignal to calculate a relative flow split between the first and secondexhaust legs.
 5. The exhaust aftertreatment system according to claim 1,wherein each of the first and exhaust valves is an exhaust throttle. 6.The exhaust aftertreatment system according to claim 1, wherein theelectronic control module is configured to operate in a compensationmode in which the first and second exhaust valves are positioned toprovide substantially equal amounts of exhaust mass or exhaust volumethrough each of the first and second exhaust legs.
 7. The exhaustaftertreatment system according to claim 1, further comprising aparticulate filter positioned upstream of the SCR in each of the firstand second exhaust legs.
 8. The exhaust aftertreatment system accordingto claim 7, wherein the control module is configured to operate in aregeneration mode in which the first and second exhaust valves arecontrolled to route substantially more exhaust through one of the firstand second exhaust legs than the other of the first and second exhaustlegs to regenerate the particulate filter in that one exhaust leg. 9.The exhaust aftertreatment system according to claim 7, wherein eachexhaust leg further comprises a temperature sensor positioned upstreamof the particulate filter, and the control module is configured tomonitor signals generated by the temperature sensors and adjust any ofthe first and second exhaust valves to a position where the temperaturereaches a predetermined level.
 10. The exhaust aftertreatment systemaccording to claim 7, wherein each exhaust leg further comprises atemperature sensor positioned downstream of the particulate filter, andthe electronic control module is configured to monitor signals generatedby the temperature sensors and adjust any of one the first and secondvalves to a shut-off position when a monitored temperature is greaterthan a predetermined level.
 11. The exhaust aftertreatment systemaccording to claim 1, wherein flow measurement device is one of adelta-P based device, a hot-film type device, a vortex shedding typedevice, and an ultrasonic type device.
 12. A method of exhaustaftertreatment for an exhaust system of an internal combustion engine,said exhaust system having plural exhaust legs in parallel with oneanother and including a selective catalytic reducer (SCR), and each saidexhaust leg including a controllable exhaust valve, said methodcomprising: flowing exhaust gas through the plural exhaust legs;detecting an exhaust gas flow characteristic in at least one of saidplural exhaust legs; determining amounts of exhaust gas flow in theplural exhaust legs based on said detected exhaust gas flowcharacteristic; and individually adjusting the exhaust valves in each ofthe plural exhaust legs based on the determined amounts of the exhaustgas flow.
 13. The method of claim 12, wherein the determined amounts arerelative amounts among the plural parallel exhaust legs.
 14. The methodof claim 12, wherein each of the plural exhaust legs includes aparticulate matter filter, and the method further comprises: monitoringsoot loading in each of the plural exhaust legs; if the monitored sootloading of one of the exhaust legs reaches a predetermined regenerationthreshold, controlling the exhaust valves to force a greater amount ofthe exhaust flow through the exhaust leg exceeding the predeterminedthreshold than the other exhaust legs to maintain a target regenerationtemperature at an inlet of the particulate matter filter of that exhaustleg.
 15. The method of claim 14, further comprising dynamicallycontrolling the exhaust valves to maintain said temperature at an inletof the exhaust leg being regenerated above said target regenerationtemperature.
 16. The method of claim 12, further including controllingthe exhaust valves at light engine load to maintain a temperature ineach exhaust leg at or above a target minimum temperature.
 17. Themethod of claim 12, wherein an SCR is provided in each of the pluralityof parallel exhaust legs.
 18. A multi-leg exhaust aftertreatment system,comprising: plural parallel exhaust legs adapted to receive exhaust froman internal combustion engine, each said exhaust leg including a dieseloxidation catalyst (DOC), a diesel particulate filter (DPF), and anindependently controllable exhaust valve; an exhaust flow sensorpositioned along at least one of the plural exhaust legs and configuredto sense an amount of exhaust flowing in that exhaust leg and generate aexhaust flow signal indicative of said amount; and an exhaust valvecontrol module connected to the exhaust flow sensor to receive saidexhaust flow signal and generate control signals for each of theindependently controllable exhaust valves based on the exhaust flowsignal.
 19. The multi-leg aftertreatment system of claim 18, furthercomprising a selective catalytic reducer (SCR) downstream of the pluralexhaust legs.
 20. The multi-leg aftertreatment system of claim 18,further comprising a soot load estimator (SLE) that determines sootloading estimates for each DPF based on the sensed exhaust flow.
 21. Themulti-leg aftertreatment system of claim 20, further comprising aregeneration module that determines which DPF to regenerate in theplural exhaust legs based on estimate values determined by the SLE. 22.The multi-leg aftertreatment system of claim 21, wherein the signalsgenerated by the exhaust valve control module cause more exhaust gas toflow in the exhaust leg to be regenerated than in other of the pluralexhaust legs.